Most razor blades on the market are produced by applying thin film hard coatings to sharpened stainless steel substrates. These coatings are typically deposited on blade edges by Physical Vapor Deposition (PVD) techniques which include vacuum conditions where raw materials, referred to as target materials, i.e., the material that is going to be deposited, are generally in solid form. Common PVD techniques incorporate processes such as sputter coating or Pulsed Laser Deposition (PLD).
Thin hard coatings have certain roles and advantages. One advantage is that the hard coatings generally strengthen blade edges, particularly those with slim profiles, by providing reinforcement to the edges, thus protecting the edges from excessive damage during shaving.
Additionally, the PVD-formed coating generally adheres well to the surface of the blade edge, in turn providing an adequate surface for subsequent depositions of coatings (e.g., Teflon® or telomer coatings or other polymeric material coatings).
However, a major disadvantage of the current PVD processes is that the PVD processes are predominantly “line of sight” processes, such that the best quality coatings are deposited on flat surfaces or components. With PVD processes, in order to get optimal coverage over three dimensional shaped components, the components are typically rotated and planetated in the vacuum chamber. While these additional steps of rotating and planetating assist in producing more uniform coverage, the quality of the coatings produced is compromised since, as known in the art, coating density degrades fairly rapidly with an angle of incidence and since the PVD process is, by its nature of deposition, known to yield a film or coating with columnar growth. A general description of such columnar growth of films may be found in J. A. Thornton, Ann. Rev. Mater. Sci 7, 239 (1977), entitled “High Rate Thick Film Growth.”
PVD-formed coatings are capable of being shaped simultaneously with deposition in that there is no need to stop the deposition process so as to allow the ultimate tip of the blade edge or the blade tip to be formed optimally for cutting hair without cutting or nicking the skin. Thus, to increase the density and limit the film columnar growth, oftentimes current PVD sputtered films or coatings are deposited with high bias, optimizing the mechanical properties of the thin film. However, a disadvantage of high bias is that it may cause the ultimate blade tip to end up being too sharp (e.g., in that there is little to no coating in the blade tip area), posing a limitation in obtaining an optimum tip radius which is desirable, as will be described below.
Consequently, uniformly coating razor blade edges has become challenging in the prior art. With an angle of incidence oftentimes being small (e.g., 15 degrees or less), even with an applied high bias, the prior art coatings on blade edges are generally still columnar with low density having compromised mechanical properties and film roughness.
Furthermore, an inherent problem of depositing coatings using vacuum techniques in general is that the coatings on the blade edges may produce a blade tip radius value which may be too great for a shave-able blade edge and may have undesirably high wool felt cutter forces (e.g., the lowest cutting force obtained from cutting the wool felt is high). In general, cutter forces are measured by the wool felt cutter test, which measures the cutter forces of the blade by measuring the force required by each blade to cut through wool felt. Each blade is run through the wool felt cutter a certain number of times (e.g., 5) and the force of each cut is measured on a recorder. The lowest value is defined as the cutter force.
For a blade edge to effectively shave hair (e.g., shaving ability or shave-ability) it is generally known that the wool felt cutter forces advantageously may need to be less than about 1.6 lbs coinciding with a blade tip radius value of about 20 nm. A description of wool felt cutter forces and blade tip radius may be found in US Patent Publication No. 2007/0227008, entitled “Razors” published on Oct. 4, 2007, U.S. Pat. No. 5,056,227, entitled “Razor Blade Technology” issued on Oct. 15, 1991, and in U.S. Pat. No. 5,048,191, entitled “Razor Blade Technology” issued on Sep. 17, 1991, the aforementioned assigned to the assignee hereof.
Additionally, the known prior art processes, such as PVD, are generally limited to coating no more than an upper portion of the blade flanks and generally not covering the entire blade flank. In addition, the deposited coating thickness decreases the further away from the ultimate tip (e.g., closer to the blade body) such that the thickness may generally be close to zero towards the end of the blade flank.
It may however be desirable to utilize the one process at one time to not only coat a portion of a blade flank but to uniformly coat an entire blade flank and also a portion(s) or the entire blade body as well, where a razor blade generally has a blade body and two blade flanks.
Hence, there is a need to better develop the process of coating razor blades to improve the quality (e.g., uniformity and denseness) and coverage of the coating while maintaining shave-ability.